Alpha-lactalbumin modifies the activity of galactosyltransferase in such a way that it inhibits the transfer of galactose from UDP-galactose to N-acetylglucosamine either free or linked as a terminal sugar of a glycoprotein, but facilitates the transfer to glucose or myo-inositol. To understand the modulation of galactosyltransferase activity essential for generating specific cell surface antigenic determinants, we have first isolated and characterized cDNA clones corresponding to Alpha-lactalbumin. Isolation and sequence analysis of the cDNA clones corresponding to the galactrosyltransferase is now essential in understanding this molecular mechanism of the modulation of the transferase.